Triode
XGPA™
Pritchard’s patent US 5,434,536 provides
the beginning of the grid-plate action solid state technology. Technically
this is an emulation, but the term “tube emulation” has been so
incredibly misused and hyped that it has gained a terrifically negative connotation.
Consequently, Pritchard now avoids its use. The grid-plate action has three
important features ranked in their relative importance: the non-constant plate
resistance, the conduction of the grid, and the non-constant voltage gain.
The non-constant plate resistance provides the major character needed to emulate
the most popular triode, the 12AX7 because its voltage gain is very nearly
constant. The plate resistance increases radically as the plate current decreases
as one can see from the low slope of the plate curves. This action is commonly
known as “soft clip”. Of course, this only works on the more positive
clipping region. The clipping on the negative side is largely a function of
grid conduction and the nature of the grid circuit. Finally, at the extremes
of operation, the 12AX7 voltage gain becomes lower.
The plate characteristic provide more than
clipping; they also generate harmonics prior to clipping and add body. Although
engineering audio paradigms would ban harmonics completely, harmonics do add
to the artistry. This may be explained by noting that the typical engineered
amplifiers distort “too fast” for musicians. The engineered amplifiers
typically use negative feedback to suppress harmonic generation prior to clipping
at the expense of much more rapid harmonic creation upon clipping. Musicians
prefer having harmonics rise over a substantial input range even prior to
clipping. The plate characteristic is the dominate source of pre-clipping
harmonics.
As Pritchard’s patent US 5,434,536 shows, Pritchard pursued two concepts
to implement both the plate characteristics and the voltage gain characteristics
and built prototype amplifiers using both. One concept was quite exacting
and modeled the vaunted “flat-plate” Telefunken 12AX7 closer more
exactly than other manufacturers’ 12AX7's. However, musicians preferred
amplifiers made with the other concept. This concept exaggerated the harmonic
production prior to clipping.
The accompanying figures show the plate characteristics of a 12AX7 and the
XGPA™ triode “plate” characteristics. The plate or output
characteristic shows the relationship between the plate or output current
as a function of the plate or output voltage. Since this not a diode, there
are many curves, each for a different input value. The operation of can be
estimated by drawing a diagonal line that is high on the left and low on the
right. This diagonal line crosses the various plate or “plate”
curves. Notice that the distances between the crossings is quite similar for
the 12AX7, but are increasingly greater when moving to the left for the XGPA™.
This indicates that the XGPA™ injects more harmonics into the signal
than a 12AX7 - and for musicians, that is a good thing.
Push-Pull
Pentode XGPA™
Push-pull pentodes with their phase splitters are well known amplifiers. However,
push-pull amplifiers have another important property that is not popularly
known and is suppressed by the engineering amplifier paradigm. Reviewing push-pull
amplifier operation, a push-pull amplifier uses a phase splitter to create
two signals that are mirror images of each other. Then the push-pull pentodes
amplify two signals these mirror-image signals. The output transformer combines
the mirror-image signals to produce a single signal for the speaker or speakers.
The other property occur when the mirror image signals have the same signal
added to both. Although the uncombined same signals are cancelled in the output
transformer, the inherent non-linear nature of the pentodes also combine these
signals in a way that they can be combined in the output transformer. This
process is quite similar to the way that push-pull amplifiers produce odd
harmonics and suppress even harmonics.
The importance of this combining property depends upon the nature of the signal.
If the signal is very low subsonic, then it will be probably heard as compression
or possibly like tremolo. If the same signal is audible, then level changes
are heard as additional notes, hence fatter. This explains why the same amplifier
sounds differently in America than in Europe. In this case the same signal
is the ripple from the power supply. In America the ripple is 120 Hz while
in Europe it is 100 Hz. More modern amplifiers have larger power supply capacitors
to reduce the ripple and to make the amplifier more “engineered”
and consequently thinner and less artistic.
The reason that engineers use larger power supply capacitors is to minimize
hum in the output. Output stage hum comes from the imbalance in the output
tubes. Since tubes are only balance so-so and also drift apart, the hum can
become sizeable with small capacitors. Pritchard Amps has solved this dilemma
by creating balanceable circuitry to both minimize hum and maximize the fat.
